Physical Chemistry 3: — Chemical Kinetics — - Christian-Albrechts ...

2.1 Definitions and conventions 12
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There are only three possible cases for the molecularity (one example for each
case):
• monomolecular (= unimolecular) reactions:
CH 3 NC → CH 3 CN (2.19)
• bimolecular reactions:
O+H 2 → OH + H (2.20)
• termolecular reactions:
H+O 2 +M→ HO 2 +M (2.21)
• Events where more than three species come together in the right configuration
and react are extremely unlikely and do not play a role.
I Caveat: The order of a reaction can be integer or non-integer. However, the molecularity
can be only 1 or 2, or at the most 3.
2.1.5 Mechanisms of complex reactions
A stoichiometric formula rarely describes the reactive events happening at the microscopic,
molecular level. It is, for example, completely impossible that the combustion
of 1 -heptane molecule with 11 molecules of O 2
-C 7 H 16 +11O 2 → 7CO 2 +8H 2 O (2.22)
in an internal combustion engine occurs in a single collision and gives 15 product molecules.
Instead, the net reaction is the result of a complex sequence of a very large
number (thousands, in this case) of unimolecular, bimolecular, and termolecular elementary
reactions.
The identification of the individual elementary reactions, the determination of their rates
as a function of and , andtheverification of the ensuing reaction mechanism is one
of the main tasks in the field of chemical kinetics.
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Examples for complex reaction systems:
• H 2 /O 2 system (Fig. 2.2),
• Combustion of CH 4 (Fig. 2.3),
• NO formation and removal in flames (Fig. 2.4).